Auto Parts Cheaper Blog

2005-2007 Ford Mustang Smart Junction Fuse Box Problems Symptoms and Cures Including Schematics.

The Original Problem

Do you have a 2005-2007  Mustang. Do you have annoying electrical problems that are driving you crazy?

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Although each Mustang is different and can either show just one or many of these symptoms, the list below is the most common, symptoms we know of, but does not describe every symptom. The only way to know if your SJB is bad with 100% Accuracy is to have it tested by a qualified mechanic with the proper equipment.


The SJB is on the passenger Kick Panel


Note: All Smart Junction Boxes new or used must be programmed to your individual mustang to work properly.


Common Symptoms of a bad SJB. (this is a broad range of symptoms, your vehicle may only experience one or more symptoms at a time).

  • One or More Brake lights is stuck on at all time.
  • Battery Drains Quickly When Car Sits for a few days
  • Headlights are on bright all the time.
  • Turn Signals or parking lights are stuck.
  • Power Windows do not work
  • Instrument cluster malfunctions
  • Dash lights quit working.


The original Smart Junction Box installed in these Mustangs had 2 major problems.

Problem 1: The windshield leak problem. These cars had a bad seal from the factory that allowed rain to drip down on the bottom of the windshield. The water leaks directly into the SJB box, and ruins it.

Problem 2: Poorly engineered part when the car was new. These circuit boards were poorly made, and after a few years of heat and cold, they fail to function any more. There are no repairable parts in a SJB; the whole module must be replaced.


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Normal Operation

The smart junction box (SJB) sends reference signals to the headlamp switch through circuits 1400 (TN/WH), 1401 (BK/LG), and 1402 (RD/WH). At any given time, the headlamp switch routes one of the input circuits to ground through circuit 1205 (BK).


When the SJB detects the headlamp switch in the HEADLAMPS ON position and the multifunction switch in the LOW BEAM position, the SJB sends voltage through circuit 1338 (WH) and circuit 1336 (LG/WH) to the LH and RH low beams, respectively.


The SJB sends a reference signal to the multifunction switch through circuits 1394 (WH/RD) and 1395 (RD/PK). When the multifunction switch is placed in the FLASH

-TO-PASS or HIGH BEAM position, the voltage signal is routed back to ground, internal to the SJB. When the SJB detects a request for flash-to-pass or high beams, the SJB provides ground to the high beam relay through circuit 1708 (LG/BK). When the high beam relay is energized, voltage is provided through circuits 1337 (VT/YE) and 1335 (YE/WH) to the LH and RH high beams, respectively.

Possible Causes

  • Circuit 1205 (BK) open
  • Circuit 1335 (YE/WH) short to voltage
  • Circuit 1336 (LG/WH) short to voltage
  • Circuit 1337 (VT/YE) short to voltage
  • Circuit 1338 (WH) short to voltage
  • Circuit 1394 (WH/RD) short to ground
  • Circuit 1395 (RD/PK) short to ground
  • Circuit 1400 (TN/WH) open
  • Circuit 1401 (BK/LG) open
  • Circuit 1402 (RD/WH) short to ground
  • Circuit 1708 (LG/BK) short to ground
  • High beam relay
  • Headlamp switch
  • Multifunction switch
  • BEC (Bussed Electrical Center)
  • SJB (Smart Junction Box)


sjb2 sjb3


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